Instructions for Sample preparation for Paraffin embedding PLEASE NOTE: There is no one-size-fits-all method of tissue preparation for all experimental designs. Before harvesting tissue, you need to assess your experimental design, choose the best procedure that will properly illustrate the pathology, be compatible with the further analysis being done, and best meet your experiment s purpose. If the unique goals of your particular research project require special handling procedures beyond this instruction, please do further literature research and discuss with us before you do anything with your valuable samples. Instructions for sample preparation for paraffin processing 1. Gross the samples Orientation Before dissecting and grossing your specimens, you must have clear goals in mind based on the type of specimen and the purpose of the experiment. Consider the best method to illustrate the pathology and best meet your experiment s purpose before doing anything to your valuable specimens. The following Figure-1 is a very good example of how to illustrate a lesion in a tubular sample by properly gross the tissue for best orientation during the embedding and cutting procedures. When grossing, try your best to cut the surface of interest (the part you want to see) flat and smooth with a sharp knife or blade. You can use colored inks (tissue marking dyes) to mark designated margins/surfaces/points to direct us to embed and/or cut. Figure-1. An example of how to properly gross the tissue to illustrate a lesion in a tubular sample. (Adapt KCC Path-Core Page 1 of 5
from Manual of Surgical Pathology (Third-Edition) ) Size/Dimension In order to get the samples fixed and processed properly, it is very important to keep at least one dimension of a sample no more than 4 mm, the maximum width length of the sample is 15 mm 25 mm. In another word, when you put your grossed tissue in a regular tissue cassette, the tissue should never be so thick that it touches both top and bottom of the cassette; or so large that it touches both left and right sides, or both front and back sides of the cassette. If you have a special need to submit samples which are larger than described above, you should always discuss and consult with us thoroughly prior to harvesting your samples. We will customize a special processing program to properly handle these tissues, if it is really necessary. Usage of Tissue Wrap and Foam Pads Wrap the samples which are small enough to be lost through the holes in the cassette in a specimen bag/lens paper/biopsy wrap (could be provided by our Core Facility) and folded four times to keep the samples inside the cassettes. Not doing so will result in a very high risk to lose your sample during the fixation and processing procedures. Although using a foam pad (could be provided by our facility) inside a cassette can keep those very thin samples flat during fixation and processing, it is strongly not recommended to use foam pad in every cassette. The foam pad will compress your sample, slow down the penetration of the ethanol, xylene, and paraffin, and may cause incomplete processing for the bigger samples. So, use it only when it is really necessary and use as less pads as possible. Multiple tissue embedding with/without ID You are allowed to put more than one tissue in a cassette, or we can combine tissues in multiple cassettes if they can be embedded in one block (extra charge may apply). But please do not put different type of tissue/organs or too many tissues in one cassette; it will result in much more difficulty when embedding and cutting. If you want to track your sample ID in a multiple tissue embedding sample(small scale tissue array), you can wrap the individual sample in tissue wraps with a labeled ID or use different colored inks (tissue marking dyes) to color code the sample. Please provide us a schematic diagram of the tissue arrangement for embedding direction if you are not using different inks to ID your sample. Multiple tissue embedding will save your time and other resources in the future. 2. Fix the samples (1) Fixatives There is no universal fixative that is ideal for all purposes. The following are the 2 most common KCC Path-Core Page 2 of 5
fixatives being used: 10% Buffered formalin in 0.1M phosphate buffer (the most popular fixatives) 4% paraformaldehyde in 0.1M phosphate buffer (2nd popular fixatives, should be freshly prepared (2) Sample Labeling and Fixation Label the cassettes (could be provided by our Core Facility) with pencil (strongly recommended) or special slide/cassette marker that is solvent proof (resists xylene and alcohol) (Note: cassettes must be dried for at least 5 minutes if using slide/cassette marker to avoid labels washing off). Dissect, gross the tissue, put the samples in labeled cassettes, and immerse the samples in fixative as quickly as possible. Swirl the container to ensure all tissues are completely immersed in fixative. (3) Duration of fixation Fixation is a chemical process, and time must be allowed for the process to complete. Both over-fixation and under-fixation can be detrimental and may be responsible for inappropriate results for your assays. Generally, duration of fixation depends on the size and type of the samples, temperature, and the fixative being used. The following procedures are based on properly grossed tissue and using formaldehyde fixatives (10% buffered formalin or 4% paraformaldehyde, the average penetration rate is approximately 0.5 mm/hour) at room temperature. Small specimens should be fixed for 12 hours or overnight. Larger specimens should be fixed for 24-48 hours. If the tissue or organ has a thick capsule (e.g. Kidney, big Lymph Node) or skin (e.g. mouse embryos), the fixative will not penetrate through the capsule as rapidly as it will penetrate normal tissue. Therefore, you should fix the specimens for a long time. Alternatively, you can cut the capsule or skin open on the side or use multiple punctures with a needle, to help achieve complete fixation and processing. Fixatives and dehydrants penetrate fatty tissues (such as mammary gland) slowly. This type of tissue must be cut even thinner than 2 mm to fix and dehydrate well under routine processing protocol. Or, customized processing protocol has to be used to properly process these tissues, if it is really necessary. (4) Ratio of fixative volume: Fixative volume/specimen size should be at least 15:1. (5) Decalcification Bone or massive calcified tissues cannot be cut by a regular microtome and must be decalcified prior to processing. Tissues that need to be decalcified can be submitted in the fixative (usually 10% buffered KCC Path-Core Page 3 of 5
formalin), rather than in 70% ETOH. This is the only case in which samples will be accepted in formalin. Tissues for decalcification should be submitted in a separate specimen container than any other samples which do not need decalcification. The specimen container and the paperwork must both clearly indicate which samples need decalcification prior to processing. The decalcification step could be done by the Core Facility. If it is difficult to properly gross tissue prior to decalcification, the paperwork should clearly describe grossing instructions for grossing after decalcification. 3. Submit the Samples Upon completion of fixation, samples should be rinsed with PBS (recommended) 3 10 minutes or flushed with tap water for one hour to wash away the extra formaldehyde. Then, the samples should be transferred to a sealed container (could be provided by the Core Facility) with enough 70% ethanol to cover them. Label the request ID on the container with tape. Do not label the request ID directly on the container using a regular maker, as it can be washed off by the ethanol. Bring the sample together with the signed request form to the Core Facility for processing as early as possible. If you want to paraffin process your sample, you should never store your sample in fixative or ethanol if you don't know whether it will interfere with your further experiment. Tissues embedded in paraffin will last for years. Problems with Submitted Tissue (1) Under-fixation or over-fixation Under-fixation is the worst problem to overcome once the tissues are processed and infiltrated with paraffin. Proteins and the nuclear acid will not be well preserved in under-fixed sample, which will lead to bad morphology and incorrect results of your further analysis. Over-fixation may result in masking of the epitopes or in strong non-specific background staining that can obscure specific labeling. Antigen retrieval techniques can help overcome this masking, but if the tissue has been fixed for a long period of time (i.e. over weeks or months), there may be no signal could be detected even after antigen retrieval. Please follow the instruction in the grossing and fixation sections in this instruction for properly preparation of your sample to help avoid this problem. (2) Tissue too thick or too large Fixatives and processing reagents can not penetrate inside these tissues. The tissue will not be fixed or processed well and it is often impossible to section such tissue. Again, tissue sections should be no thicker than 4 mm. (3) Small tissue without tissue wrap Please aware that most of the tissue will shrink about 1/5 of its original volume. Therefore, always wrap the samples which are small enough to be lost through the holes in the cassette in a specimen bag/lens paper/biopsy wrap and folded four times to keep the samples inside the cassettes. it is not recommended to KCC Path-Core Page 4 of 5
use foam pads to keep you small sample in the cassettes. (4) Abuse usage of form pad The foam pad will compress your sample, and slow down the penetration of the ethanol, xylene and paraffin and may cause incomplete processing for the bigger samples. Therefore, use it only when it is really necessary. (5) Calcified Substances As a rule of thumb, any tissue submitted for processing should be easily sectioned with a scalpel blade. Bone or massive calcified tissues cannot be cut by a regular microtome and must be decalcified prior to processing, or surface-decalcification must be performed after processing(less recommended). (6) Hair and hard Foreign Material Hair and hard foreign material can dull microtome blades. Please carefully remove hair off the sample if abundant on a skin specimen or dermoid cyst. Flush the gastrointestinal tracts thoroughly to remove foreign contents. Staples, sutures and clips must be removed from tissue before submission to the facility. KCC Path-Core Page 5 of 5